1. Field of the Invention
The present invention relates in general, to communication, and in particular, to a communication intercept device using digitally programmable devices to channelize, detect, and recognize arbitrary signals. The present invention involves the use of a new and improved variable delay First-In-Tap-Out (FITO) spectral memory storage, a two-stage synthesis filter for improved dynamic range and programmable software digital receivers that use spectral synthesis rather than numerically controlled oscillators for signal demodulation.
In addition, the present invention relates to detection, demodulation, and determination of direction of signal activity, and in particular, to a signal intercept and direction finding device using general purpose commercial-off-the-shelf (COTS) digitally programmable devices to hyperchannelize radio or acoustical spectrum into spectral cells narrower than the signal structure of interest so as to detect, synthesize, and recognize arbitrary signal activity, and determine signal direction by phase determination of angle of arrival or time difference of arrival.
Overall, the present invention relates to a new and improved multiple and cascadable module device using general purpose digital processing, organized in a fashion that each module of respective digital signal processing means comprises data buffering, digital signal processing and high-speed data routing devices.
2. Discussion of the Related Art
A communication intercept device typically employs a wideband receiver that amplifies and down converts a large portion of the radio frequency spectrum. For the detection of narrowband signals, the wideband receiver output is channelized by some means such as acousto-optic Bragg cells and photo detectors, surface acoustic wave (SAW) devices and programmable digital Fast Fourier Transformation (FFT) devices.
The digital FFT based channelizer is more flexible than the acousto-optic or SAW device, and is capable of providing complex (real and imaginary) spectral segments sampled above the Nyquist rate. In many applications the channelization segments match the bandwidth of the intended receive signal, and each channelization segment is therefore connected to a digital signal processing (DSP) device. For example, a series of U.S. Pat. Nos. 5,535,240, 5,537,435 and 5,848,097 commonly entitled “TRANSCEIVER APPARATUS EMPLOYING WIDEBAND FFT CHANNELIZER AND INVERSE FFT COMBINER FOR A MULTICHANNEL COMMUNICATION NETWORK” disclose a transceiver apparatus that employs this fundamental architecture.
To reduce the number of DSP devices (e.g. digital drop receivers) required in an intercept system, strategies are used to find the spectral channels of activity. For example, U.S. Pat. No. 5,428,667, entitled “MULTI-CHANNEL CELLULAR COMMUNICATIONS INTERCEPT SYSTEM” discloses a device wherein one or more of the plurality of channelized outputs are selected for continuous demodulation. The selected channels are the cellular forward control channels that contain formatted instructions to mobile radios, establishing the immediate use of narrowband duplex channels for cellular network communication. The intent of this invention is to determine the cell channel assignments from the control channels and assign only a small plurality of drop receivers to the cell channels in immediate use. Estimates of the number of drop receivers required to satisfy the demand to a level of confidence can be determined by Poisson statistics based on channel activity and cellular call duration as those skilled in the art are well aware.
Further enhancements to wideband intercept systems have been made. U.S. Pat. No. 5,812,609, entitled “COMMUNICATION INTERCEPT DEVICE USING DIGITAL DROP RECEIVERS IN MULTIPLE TIERS” discloses a device that uses a plurality of digital drop receivers in a configuration using a plurality of tiers. This device uses fewer digital drop receivers than one per voice grade channel but allows the monitoring of all narrowband signals of interest. As exemplified in the device description, the first analog to digital converter (ADC) is connected to a first memory that stores the digitized wideband signal. A plurality of first digital drop receivers is connected to the first memory for selecting various wideband signals in a multiplexed fashion under control of a controller. The output of a first digital drop receiver is connected to a second memory for storing the medium band signals of interest. The second memory has an input connected to the output of each of said first plurality of digital drop receivers, storing the signals selected. A second plurality of digital drop receivers in a second tier is connected to the second memory for further selection of various narrowband signals of interest in a multiplex fashion under control of the controller.
Additional enhancements have been made to wideband communication devices, including dynamic signal routing and wideband gain control. U.S. Pat. No. 5,590,156 entitled “MULTICHANNEL WIDEBAND DIGITAL RECEIVER MAKING USE OF MULTIPLE WIDEBAND TUNERS HAVING INDIVIDUALLY SELECTABLE GAINS TO EXTEND OVERALL SYSTEM DYNAMIC RANGE” discloses a wideband digital receiving device that processes and converts from analog to digital representation the bandwidth in a plurality of sub-bands. The output of each sub-band is channelized using a complex FFT engine and convolution filtering. According to the device description, the convolution filter may be a so-called overlap and add digital filter, or may be a polyphase digital filter. Each digital channel signal corresponding to a digitized version of the communication signal transmitted by one of the subscriber units. Before passing the individual digital channel signals to the DSP processors, each digital channel signal output may also be processed by a sample rate adjuster that provides samples of its respective digital channel signal taken at or near a position of peak symbol amplitude. Other aspects of the invention describe amplifier attenuation factors on the wideband receivers based on received signal strength indicator (RSSI) made at the output of the channelizer by the DSP processors. As part of the communication network, forward channel control reassignment is made to subscriber frequencies, grouping like RSSI amplitude signals in frequency channels of a common wideband receiver, thereby maximizing channel path ADC performance.
Multichannel wideband digital receivers are extended to perform direction finding by angle of arrival. U.S. Pat. No. 5,815,117 entitled “DIGITAL DIRECTION FINDING RECEIVER” discloses a multichannel device capable of receiving radar pulse signals simultaneously from a plurality of antennas. Each respective antenna signal is received and analog to digital converted. A first antenna digital channelizer divides the received antenna signal into N digital frequency channels covering the received spectrum. A threshold detector means determines pulse signal presence in a respective channel and tunes a digital local oscillator (LO) to the channel frequency as reference. A plurality of digital down converters receive respective plurality of delayed antenna digital signals from a First in First Out (FIFO) buffer and respective tuning digital LO reference. The plurality of digital downconverters output tuned and filtered narrowband signals that are phase coherent. The narrowband antenna signal outputs are received by a phase detector that compares phase of the first antenna narrowband antenna signal output to all subsequent antenna narrowband antenna signal outputs. The plurality of signal phase angles and respective signal amplitudes is used in a conventional manner by an angle of arrival encoder to estimate the direction of a signal in the selected channel.
Although wideband receivers and channelizers with drop receivers and multichannel processors have been used in the past, it will be readily appreciated that improvements are needed to: (1) provide improved signal detection and direction finding of signals whose bandwidth do not match a fixed channel bandwidth, (2) provide functional isolation and graceful system degradation by re-routing data from one module processing device to another, (3) provide flexible ordering of the module processing functions, including detection, demodulation and angle of arrival processes, and (4) provide efficient digital signal processing using general purpose digital processors devices for lower cost and long term supportability.